Transformer unit 1

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If we use in our crystal receiver a detector circuit with a very high Q, then the parallel resistance of the circuit will also have a very high value.
The load impedance which we connect to the circuit, must also have about the same value.

A circuit has at 1000 kHz a Q of 1200, the induction of the detector coil is 0.2 mH.
The parallel resistance (Rp) of the circuit will have a value of  Rp= 2.pi.f.L.Q= 1.5 M.Ohm (Mega-Ohm).
At other frequencies there is another value for Rp, because both f and Q will change.

The transformer unit described here has a input impedance of 1.62 M.Ohm so it is very suitable for loading high Q circuits in crystal receivers.
I build a separate unit for it, so it can be used for several crystal receivers, and also because I only have one piece of the special input transformer (Tr1).

Transformer unit 1

Transformer unit with a input impedance of 1620 k.Ohm.
And a output impedance of 16 Ohm.

The large transformer on the left is Tr1, the other is Tr2.
The dimensions of the unit are: 18x11x10 cm.


The transformer unit is build with two transformers.

Transformer Tr1 comes from a high voltage power unit, brand "Simco Nederland b.v." and type number A257C1-6.
The transformer has a primary coil which was connected to 220 Volt 50 Hz via a 910 Ohm series resistor.
The secondary coil gives 7 to 8 kilo-Volt at 2.5 mA.
So this secondary coil has a very high impedance.

In my transformer unit the input signal is connected to the secondary coil of Tr1, and the primary coil of Tr1 is connected to Tr2.
So the primary and secondary coil have changed function, and Tr1 now works as a down transformer.

With different load resistors, I measured the following properties of Tr1:

- Input impedance
- Frequency bandwidth (-3dB)
- Efficiency
A test method for these measurements, you will find  here
The results are in the following table:

Load resistance 
Input impedance 
Frequency range 
-3 dB (Hertz)
68 524 140-4600 0.733
100 730 150-4500 0.808
150 980 150-4120 0.806
220 1338 150-3400 0.794
330 1815 160-2700 0.761

Behind Tr1 I placed a second transformer (Tr2) which transforms the voltage and impedance further down.

Tr2 is a audio transformer for 100 Volt loudspeaker systems.
It's brand is Adastra and type number 952.446.
On the primary coil, I used the 40 Watt connection, which has a impedance of 250 Ohm.
The output impedance of Tr2 is 8 or 16 Ohm, I use the 16 Ohm connection.

De complete transformer unit has the following specifications:
Input impedance: 1620 k.Ohm
Output impedance: 16 Ohm
Frequency range (-3 dB): 150--2700 Hz.
Ratio input voltage  / output voltage:  377
Efficiency: 0.71

The resistor and capacitor in the schematic are added to become a DC resistance for the unit which is the same value as the impedance, this is important to prevent sound distortion when receiving strong stations.
Over the capacitor we can measure a DC voltage, which indicates the strength of the received station.
Use a voltmeter with at least 10 M.Ohm resistance for this.

The ground connection of all coils and the iron core of TR1 are all connected to each other, this prevents picking up hum.

In this setup (secondary coil of one transformer connected to the primary of another transformer), we must multiply the efficiencies of both transformers, to become the efficiency of the complete unit.

when I connect a  driver unit to the output of the transformer unit, a 1 kHz test tone on the transformer unit input with a amplitude of 1 mV peak-peak can be easily heard.

We can still use the transformer unit as a up transformer, for instance by connecting a signal generator to it's output.
This is very dangerous, because the voltage at the input of the transformer unit can now easily reach thousands of Volts.
But for experiments this can be useful, that's why I used a 4kV type for the 4n7 capacitor, so it can not be destroyed so easily.

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